Helmet with a magnetostrictive actuator

ABSTRACT

A magnetostrictive actuator comprises a rod of giant magnetostrictive material, biasing permanent magnets arranged at each end thereof and coaxially therewith so as to pass a magnetic field through the rod. An energizing electromagnetic coil is connected to an energizer so as to superimpose on the magnetic field passing through the rod an alternating magnetic field to cause variation in the length of the rod in response thereto. The permanent magnets are each in the form of a disc having a cross-sectional area substantially greater than that of the rod and are spaced from the ends of the rods by material of low magnetic permeability. The magnetic field in the rod is thereby substantially linear along the length thereof.

This application is a divisional of co-pending U.S. Patent ApplicationSer. No. 10/913,582, filed on Aug. 6, 2004.

FIELD OF THE INVENTION

This invention relates to a helmet or protective hat having amagnetostrictive actuator especially suitable for audio frequency useand to audio output and communication devices which may employ such anactuator.

BACKGROUND TO THE INVENTION

It is known that a panel may be made to function as a loudspeaker byattaching to the panel an audio frequency actuator which can transmitacoustic signals into the panel. WO92/03024 discloses one such panel.WO98/52289 suggests the use of standard trim panels in a motor vehicleas a means of radiating the output of a radio, tape player or the like,listing magnetostrictive, electromagnetic, electrostatic and micromotortransducers as possible for the job, although preferring piezoelectrictransducers.

While such panels can be made to operate as satisfactory loudspeakers,they all depend on attachment of the actuator to the panel, and theavailability of mains power or a vehicle alternator output to drivethem, for various reasons. Piezoelectric devices require high voltagesand relatively large currents to produce suitable levels of audio power,while electromagnetic devices, although operating at low voltages,require very high currents to produce sufficient power to produceaudible output from the panels. Magnetostrictive devices can generate asubstantial force at a relatively low voltage, but have previously beenimpractical for portable domestic uses for two reasons. Firstly, thecurrent consumption to generate the necessary magnetic fields to causechange in length of the giant magnetostrictive material (GMM) isrelatively high, and secondly, the cost of the GMM would be prohibitive.

It has been found that simply scaling down conventional magnetostrictiveactuators does not produce satisfactory audio output, with high levelsof distortion or very low volume.

SUMMARY OF THE INVENTION

The present invention provides, in one aspect, a magnetostrictiveactuator, compnsing a rod of giant magnetostrictive material, a biasingpermanent magnet arranged at each end thereof and coaxially therewith soas to pass a magnetic field through the rod, and an energisingelectromagnetic coil connected to energising means so as to superimposeon the magnetic field passing through the rod an alternating magneticfield to cause variation in the length of the rod in response thereto,wherein the permanent magnets are each in the form of a disc having across-sectional area substantially greater than that of the rod and arespaced from the ends of the rods by material of low magneticpermeability, such that the magnetic field in the rod is substantiallylinear along the length thereof.

The rod may comprise two or more pieces of GMM located end-to-end. For agiven rod, it has been found that the axial distance between the twomagnets is suitably approximately equal to or less than the diameter ofthe magnets.

The body of the actuator is formed of a non-magnetic material, forexample a 10 zinc alloy, rather than steel, as in conventional magnetostrictive actuators. Thus, a magnetic return path is not provided, andit has been found that this also helps to en hance the linearity of themagnetic field through the GMM.

The magnets preferably have a diameter at least three times the meandiameter of the rod, and more preferably at least four times. Thethickness of the magnets is 15 preferably substantially less than thediameter of the magnets. The spacing of each magnet from the end of therod is preferably a distance at least equal to the thickness of themagnet. The aim is to ensure that none of the GMM is magneticallysaturated, and that the magnetic field through the rod is as nearparallel as possible to the longitudinal axis of the material.

The magnets are suitably of the neodymium-iron-boron high energy type.Other types of magnet could be used, but the field strength may belower, and it is therefore less likely that there would be a linearfield throughout the length of the GMM rod. The GMM rod is preferablyless than 15 nm, more preferably 12 mm. or less, and the width ispreferably 4 mm or less. Typically, the rod will be of square section,say 4 mm, by 4 mm.

It has been found that the use of large-diameter magnets gives asubstantially linear magnetic field throughout the length of the GMMrod, thus ensuring that the magnetic field influences the greatestpossible amount of the GM material, thereby maximising the response ofthe actuator to the applied energising field. The non-magnetic spacersensure that the amount of GMM required is minimised by avoiding magneticsaturation at the ends of the GMM rod.

It will be understood that the reference to “disc” is not intended toimply that the magnets must be circular in plan; other shapes may beequally effective, including square.

The invention further provides an audio device comprising amagnetostrictive actuator according to the invention, adapted to belocated on a surface into which the actuator can couple acoustic wavesinto the surface, the audio device being such that the greater part ofthe mass thereof is supported by the actuator.

Another aspect of the invention provides a self-contained device forinducing acoustic waves into a panel such that the panel radiatesaudible sound, the device comprising a casing containing a power source,an audio frequency signal input, and an amplifier connected to the powersource and to the signal input and providing an output signal to anaudio frequency actuator adapted to couple acoustically with the panelwhen the device is brought into contact therewith, thereby producing anamplified audio output from the panel.

Yet another aspect of the invention provides a self-contained device forinducing acoustic waves into a panel such that the panel radiatesaudible sound, the device comprising a casing containing a power source,an audio frequency signal generating means, and an amplifier connectedto the power source and to the signal generating means and providing anoutput signal to an audio frequency actuator adapted to coupleacoustically with the panel when the device is brought into contacttherewith, thereby producing an amplified audio output from the panel.The audio frequency actuator is suitably a magnetostrictive actuatorcomprising a magneto strictive element and an electromagnetic coilassociated therewith, the coil being connected to the amplifier so thatthe output signal causes the element to deliver an output force which ismodulated by said signal, and wherein means are provided fortransmitting the output force to the panel when the device is broughtinto contact therewith.

More preferably, the actuator is of the type hereinbefore defined. Thesignal generating means may be a player device for reproducing arecorded audio frequency signal, for example a recording tape player, aCD player, a DVD player, or a solid state memory device. Alternatively,the signal generating means may be a radio receiver, for example abroadcast radio receiver or a receiver for a locally radiated radiosignal, for example providing a wireless connection from a local signalsource.

Another aspect of the invention provides an audio communications devicehaving a microphone and an audio output circuit connected to an audiofrequency actuator adapted to induce acoustic waves into a panel whenthe device is brought into contact therewith such that the panelradiates audible sound.

The communications device of the invention preferably includes a radioreceiver/transmitter connected to the microphone and to the audio outputcircuit, and may, for example, be a cordless telephone handset of thetype linked by radio (digital or analogue) to a base station which iswired to the public switched telephone network(PSTN). Cordlesstelephones of this Peneral type are common in houses and offices.Alternatively, it may be an independent radio telephone, for example acellular radio telephone. There have been some concerns about the use ofsuch telephones, especially cellular telephones, and their possibleeffect on users' brains, since it is necessary to hold a source ofmicrowave radiation, albeit weak radiation, close to the user's head tohear the output of the internal loudspeaker. While various “hands-free”solutions have been proposed, most require some electrical link betweenthe telephone and an earphone, and this is thought by some to act as anantenna radiating towards the user's head. Where the communicationsdevice of the invention is a telephone handset, it can be arranged to beused in one mode conventionally, by bringing it into close proximitywith the user's ear, or in an alternative mode as a loudspeakingtelephone simply by placing it on a horizontal surface such as a tableor similar panel, or by attaching it temporarily to an upright surfacesuch as a window, for example by a separate mounting bracket, by meansof a suction attachment device as disclosed in our-pending applicationGBOI 16303.9, by double-sided adhesive tape or using temporary adhesivematerial. The device needs to be held in intimate contact with thesurface for satisfactory acoustic coupling with the panel to occur, andthe body of the handset will be designed to ensure that intimate contactcan occur when the handset is simply resting on a horizontal surface,for example. An additional advantage of this property is that“conference calls” in which several people in the same room, forexample, can participate in the same telephone call at the same time canbe conducted without the need for special additional equipment.

Since the actuator will still induce sound waves into the body of thehandset, the handset may still be used in conventional manner whenprivacy is essential and conditions permit “hands-on” use.

Further advantages arise from this property of the device. Since thebody of the handset radiates sound waves of sufficient amplitude topermit the output to be heard when the handset is held near to the ear,precise positioning of the handset relative to the ear becomesunnecessary-with conventional handsets having holes in the body next tothe loudspeaker, alignment of the holes with the ear is necessary toachieve adequate sound volume, and this position may not be convenientfor the comfort of the user. A further advantage is that, since holesthrough the casing are no longer necessary, the casing can more readilybe made waterproof or at least water resistant. This has advantages whenapplied to communications devices used in adverse environments, forexample those used by the military or the emergency services. A furtheradvantage is that the absence of feedback despite the close proximity ofthe microphone to the surface radiating the sound means that it is notnecessary to employ special controls, whether software or hardwarebased, to avoid feedback problems, and the handset may be made smallersince the microphone—speaker separation distance is no longer important.It will be seen that the handset according to one embodiment of theinvention can be used in three main ways: spontaneously, as a hand-helddevice or one that permits communication through touch, or by simplyresting the handset on a surface; shortterm, by temporarily attachingthe handset to a surface using a sucker attachment, for example, or byusing double-sided adhesive or a temporary adhesive; or permanently, byadhering the device to a surface with a permanent adhesive, for examplefor car use where it is desired to ensure that any use will be on ahands-free basis, and not handheld and therefore contrary to law. Thelatter case may be useful where the employer has a duty to ensure thatthe employee conforms with the law, and may have a legal liability wherethere has been failure to comply. In another embodiment of theinvention, the device is provided with a two-part attachment means,having a first part permanently attached to a surface which is toradiate sound, for example a desk-top or a window, and a second partconnected to the actuator, the two parts being connectable togethertemporarily by a twist-locking or spring docking arrangement, forexample.

In another embodiment, the device comprises means for connection to anexisting telephone, for example a cord, or a plug connector directlyengaging a socket in the telephone handset body (or vice versa). In thelatter case, the device of the invention can serve as a “dockingstation” or may be arranged to couple permanently or temporarily usingany of the methods indicated above. To enhance the acoustic coupling,the device may be designed so that the greater part of the mass thereofis carried by the actuator and therefore serves as a reaction masstherefor. For example, the actuator may be provided with a “foot”coupled directly to the GMM element and adapted to serve as the solesupport for the device on the surface. The reaction mass will theninclude all those elements in the device which are rigidly supported bythe actuator (springmounted elements such as batteries will notcontribute to the enhancement of the coupling of the acoustic waves intothe surface).

The magnetostrictive actuator may comprise a bar of magnetostrictivematerial, an electromagnetic coil surrounding the bar, a first permanentmagnet located at one end of the bar with the south pole thereofdirected towards the bar, a second permanent magnet located at the otherend of the bar with the north pole thereof directed towards the bar,incompressible spacer means located between each magnet and the bar,said spacer means being of a material of low magnetic permeability, andmagnetic circuit means extending from the outwardly-directed pole of thefirst magnet to the outwardlydirected pole of the second magnet.

Acoustic coupling may be improved by providing a thin layer of greasebetween each adjacent pair of components in the actuator, e.g. betweenthe GMM bar and the spacers, and between the spacers and the magnets.

With this configuration, it is possible to construct an effectivemagneto strictive actuator which has a very small size and weight, forexample less than 75 g, and using only a small, and therefore lesscostly, quantity of magnetostrictive material. For example, a length of12 mm or less is suitable, and preferably 10 mm, or less, with athickness of 4 mm or less. In an audio device, such an actuator canstill produce sufficient audio output to permit it to be heard by anumber of people in a typical office or home environment, for example.

According to a further aspect of the invention, a rear-view mirror unitfor a motor vehicle comprises a mirror mounted as a front face of ahollow casing, the casing having audio input means connected to an audiotransducer within the casing.

The audio transducer may be a conventional loudspeaker, for example amoving coil type of device, but is preferably a magnetostrictiveactuator in contact with the rear face of the mirror and arranged tocouple acoustically with the mirror thereby outputting sound therefrom.

The audio input means, which is preferably connected to the actuator viaan amplifier, may comprise a connecting socket, for example to permitconnection, via a suitable lead and plug, to a mobile telephone,permitting hands-free operation thereof in a motor vehicle, the mirrorunit serving as a loudspeaker. The mirror unit may also include amicrophone.

Alternatively, the mirror unit may include radio receiving means, forexample of the type permitting connection to a mobile telephone or thelike via a short-range radio link, one example of which might be thatknown under the trade mark Bluetooth.

Another way of connecting to a mobile telephone would be by way of aninfrared link, for example of the type used to connect peripheraldevices to computers.

The casing may be provided with a microphone connected to an audiooutput means such as a connecting socket for a wired connection to anexternal device such as a mobile telephone, or a wireless connection.The microphone may also be connected to electronic control circuitswithin the casing to permit, for example, voice-actuation of the unit.

Electronic control means within the casing may be alternatively oradditionally be connected to a touch-operated device on the casingwhereby a user can control operation of the device by touching thetouch-operated device. The touch-operated device may be a simplepush-button switch, or a non-moving switch such as an inductive orcapacitive sensor.

The touch-operated device or button is suitably configured as asingle-push button positioned towards the left hand side or middle lowercentre of the casing, in the case of a right-hand drive car, to bepushed to acknowledge or open a call, when used in conjunction with amobile telephone, or when pressed in the receive mode will cause accessto voice dial as a voice prompt. To receive a call, means may beprovided on the casing to indicate an incoming call, and the user willthen press the button on the casing to open the call. Cancelling thecall is achieved by pressing the same button. To initiate a call,pressing the button once may be arranged to open a tone that uses avoice tag attached to a recipient's name. The device may be arranged tomimic the voice tag to cause the telephone to dial the recipient.

Noise suppression circuitry and software may be incorporated into thedevice.

The mirror unit casing may be provided with an earpiece providing audiodirect into a user's ear, the earpiece being connected to the audioinput means via a wire which is provided with a rewind mechanism so thatthe earpiece may be pulled out of the casing and inserted into the earto permit private listening, and then permitted to retract again underthe influence of the rewind mechanism when no longer required. Extensionof the earpiece may be arranged to disconnect the audio input from theaudio actuator automatically. Similarly, a microphone may be providedwhich can be pulled out from the casing and, for example, clipped to theuser's clothing, to permit speech at a lower level.

Other panels within the passenger compartment of a motor vehicle may becombined with the actuator to provide a sounder for reproducing soundaudibly within the passenger compartment. For example, the sun visor forthe front windscreen or windshield may be used, or a transparent screenoverlying the instrument panel of the dashboard.

A still further aspect of the invention provides a communications systemcomprising a base station including a microphone and means forbroadcasting a local radio signal carrying the audio output of themicrophone, and a plurality of remote units, each remote unit beingattached to a panel remote from the base station and having a radioreceiver for selectively receiving the broadcast signal and foroutputting an audio signal to an actuator arranged to coupleacoustically with the panel, thereby rendering the audio signal audiblein the region of the panel.

This provides a very flexible and less intrusive public address system,where local surfaces such as windows, doors, wall or ceiling panels,desks or other furniture can be made to radiate an audio signal. Anotherapplication would be the distribution of announcements at a public eventwithout the need for high volume loudspeakers, which tend not only todirect sound where it is wanted, but also when it is not wanted.

The radio receivers may be individually addressable, for example bymeans of packet radio distribution techniques, permitting messages to besent to individual locations. A possible application for this would bethe provision of individually selected music to different tables in arestaurant, where the use of an actuator on the table surface ensuresthat the choice of music is heard at acceptable volume by those at thetable, but does not intrude on those sitting at adjacent tables. Anotherpossible example would be inmuseurns and the like, where the actuatorscould be associated with a clipboard carried by visitors, for exampleschoolchildren taking notes, while announcements could be addressed toindividuals or groups.

Yet another aspect of the invention provides a display device comprisinga surface on or behind which information is displayed, an audiofrequency actuator acoustically coupled to said surface for inducinginto said surface acoustic waves causing the surface to radiate audiblesound, and storage and playback means connected to said actuator forreplaying a recorded audio signal.

Thus, for example, a public advertising display may be arranged todeliver an audible message to supplement or reinforce the messagedisplayed visually on the display surface, which may be a static displaysuch as a poster, or a dynamic display such as a video display, forexample. The display device responds to the presence of a person inproximity to the device; what constitutes proximity will be determinedby the location and the type of information being displayed and madeaudible, for example. The display may be a free-standing board type ofdisplay, or a display behind glass, for example behind a shop window,which serves as the surface to which the audio frequency actuator iscoupled.

Sensor means may be provided for detecting the presence of a person inproximity to the surface and for causing operation of the storage andplayback means to send an audio signal to the actuator in response tosaid detection. Alternatively, the display device may be provided with aswitch operable by the viewer, or may be arranged simply to operate on aloop.

In yet another aspect of the invention, a helmet or hard hat has anaudio frequency actuator acoustically coupled therewith, and is providedwith means for supplying an audio frequency drive signal to theactuator.

The means for supplying the audio frequency signal may be a link to aseparate radio communications device, or it may comprise a radioincorporated into the helmet. Such a helmet may be useful for firefighters, permitting messages to be heard clearly in adverse conditions,since the helmet radiates sound around the head, permitting it to beheard without blocking out, or being blocked out by, ambient sound.

In an alternative embodiment, a helmet or protective hat comprises ashell having a headband mounted therein for locating the helmet or haton the wearer's head, the headband carrying an audio frequency actuatorconnected to means for supplying an audio frequency drive signalthereto, the actuator being positioned so as to engage the wearer'shead, in use, to couple acoustic energy directly into the wearer'sskull.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which illustrate exemplary embodiments of theinvention:

FIG. 1 is a cross-sectional view of the central part of an actuatoraccording to the invention;

FIG. 2 is an enlarged view corresponding to the principal elements ofFIG. 1, with typical magnetic lines of force superimposed thereon;

FIG. 3 is a cross-sectional diagram of an audio device according to oneembodiment of the invention;

FIG. 4 is a similar diagram of an alternative embodiment of audiodevice;

FIG. 5 is a cross-sectional diagram of an audio communications deviceaccordiing to another embodiment of the invention;

FIG. 6 is a cross-sectional diagram of a rear-view mirror in accordancewith the invention;

FIG. 7 is a front view of an alternative form of rear-view mirror for avehicle;

FIG. 8 is a view of an interior of a motor car with a magnetostrictiveactuator installed therein;

FIG. 9 is a plan view of a building incorporating a communicationssystem in accordance with yet another embodiment of the invention;

FIG. 10 is a diagrammatic view of an alternative form of communicationssystem;

FIG. 11 is a schematic side elevation of a display device in accordancewith the invention;

FIG. 12 is a schematic sectional view through a helmet in accordancewith another aspect of the invention; and

FIG. 13 is a schematic view of a hat in accordance with yet anotheraspect of the invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The magnetostrictive actuator shown in FIG. 1 comprises a casing 1, forexample of a non-magnetic material, in which is located a rod 2 of giantmagnetostrictive material (which for convenience may comprise two piecesof the GMM, end-to-end), with a non-magnetic spacer 3, for example ofaluminium or a ceramic material capable of transmitting force, disposedat each end thereof to space the GMM rod 2 from a pair of disc magnets 4of substantially greater diameter than the width of the rod. A plasticscore 5 serves to locate and hold the GMM rod 2 and spacers, and carriesan electromagnetic winding 6 which is connected via wires (not shown) toa separate energising signal source. A foot 7 transmits the forceproduced by the actuator to the desired location. A spring clip 8 heldin grooves around the inside of the casing 1 and the foot 7 holds theassembly together and imposes a mechanical pre-stress on the GMM rod 2.

As may be seen more clearly from FIG. 2, by selecting disc magnets 4 ofhigh energy, such as the neodymium-iron-boron type, and having adiameter substantially greater than that of the rod 2, the lines offorce between the two magnets, although curved at the periphery of themagnets, will be substantially linear in the central region of themagnets coinciding with the area of the rod, thus ensuring the maximummagnetostrictive effect along the whole length of the rod. For a givenlength of rod, therefore, the extension achieved by a given magneticfield applied thereto is maximised.

Referring now to FIG. 3, the audio device comprises a casing 21 containsbatteries 22, typically replaceable dry cells or rechargeable cells, andan electronic circuit 23 including an amplifier powered thereby. Aninput socket 24 receives a connector plug, for example a miniature jackplug, on a connector lead to an external signal source, for example apersonal stereo player, a mobile telephone or the like. The electroniccircuit 23 within the audio device comprises a squelch circuit whichdetects the presence or absence of an input signal and switches theamplifier to a quiescent state when no signal is present to preservebattery life. The casing 21 has mounted therein a magnetostrictiveactuator 25, for example of the type described hereinbefore withreference to FIGS. I and 2. The actuator is connected to the amplifieroutput so that the magnetostrictive material changes its length inresponse to the audio frequency signal output by the amplifier. Theactuator is in intimate contact with the casing 21, which in turn can bebrought into close contact with the surface of a panel such as a table26, for example, or a window, so as to couple with the surfaceacoustically, inducing an acoustic wave within the panel. This causesthe panel 26 to radiate sound.

In the audio player illustrated in FIG. 4, a casing 30 contains a discplayer such as a Compact Disc player or MiniDisk player, showndiagrammatically at 31. The construction of such players is well-knownand is therefore not shown or described in detail. The casing alsocontains batteries 32, typically replaceable dry cells or rechargeablecells, and an electronic circuit 33 including an amplifier poweredthereby. The casing 30 has mounted therein a magnetostrictive actuator35, for example of the type described hereinbefore with reference toFIG. 4. The actuator is connected to the amplifier output so that themagnetostrictive material changes its length in response to theaudiofrequency signal output by the amplifier. The actuator is inintimate contact with the casing 30, which in turn can be brought intoclose contact with the surface of a panel such as a table 36, forexample, or a window, so as to couple with the surface acoustically,inducing an acoustic wave within the panel. This causes the panel 36 toradiate sound. It will be understood that other audio sources mayreadily be substituted for the diisc player 31, for example an FM orother radio receiver, a tape cassette player, or a solid state audiostorage and playback device.

FIG. 5 shows a cellular radio telephone comprising, in conventionalmanner, a casing 40, containing a battery 41, a main circuit board 42with electronic components 43, 44 and 45 thereon, a liquid crystaldisplay screen 46, a keypad 47, a microphone 48, 25 and an antenna 49.In place of the conventional loudspeaker, a magneto strictive actuator50 is mounted within and in contact with the casing 40, and is connectedto the amplifier section of the main circuit board, which is adapted toprovide a suitable driving output for the actuator 50.

In use, the telephone can be used in conventional manner (the firstmode), being held to the side of the face so that the microphone isadjacent to the mouth, while the actuator part of the body is adjacentto the user's ear. The actuator 50 will in these conditions cause thecasing 40 to radiate sufficient sound energy to permit the user to hearits output in the conventional way, so the exact positioning of thehandset relative to the ear is not critical. For hands-free orconference use, however, the telephone may be placed on a panel such asa table 51 (the second mode). The actuator 50 then couples the acousticenergy into the panel, causing it to radiate the sound at a levelsufficient to enable the sound to be heard in the vicinity thereof Aconventional microphone will typically have sufficient sensitivity topick up speech and other sound from the vicinity, but automatic gaincontrol may ensure that the sensitivity is adjusted to suit thecircumstances of use, so that the sound volume transmitted by thetelephone is not excessive when the telephone is used conventionally.

It will be seen that an advantage of the telephone of the invention isthat there is no need for any adaptation or adjustment to change fromhand-held to hands-free use and vice versa; the adaptation is automaticas a result of the dual mode capability of the telephone.

In the rear-view mirror unit shown in FIG. 6, the mirror 60 is held inthe open front face of a housing 61 which is mounted via a bracket 62 tothe vehicle windscreen or to the inner roof surface above thewindscreen. The mirror will typically be mounted within the housing 61in a swivel mechanism to permit it to be adjusted to avoid glare fromfollowing headlights, this mechanism is omitted from the Figure for thesake of clarity, as it has no effect on the features of the invention.The casing 61 contains an rlectronic circuit board 63 mountingelectronic components providing inter alia an amplifier, connected to amagnetostrictive actuator 64 which is mounted in intimate contact withthe rear face of the mirror 60, for example by means of adhesive. Poweris supplied to the circuit board 63 from the vehicle's electrical systemthrough a supply lead 65 which runs through the mounting bracket 62. Amicrophone 66 is mounted within the housing so as to receive sound fromwithin the vehicle's passenger compartment, and this is then connectedto the circuit board via a connecting cable 67. The circuit board 63 isalso connected via a further cable 68 to a socket 69 opening through thehousing.

The socket 69 permits connection via an external lead and plug (notshown) to a cellular telephone in such a manner that speech received bythe microphone 66 is sent to the telephone to be transmitted in theusual way, while speech received by the telephone is passed via theamplifier on the circuit board 63 to the magneto strictive actuator 64,which then couples acoustic energy into the mirror, causing it toradiate sound within the vehicle passenger compartment so that it isaudible to people sitting in the compartment. In this way, the mirrorunit can permit hands-free operation of the telephone, with the soundemanating from a location adjacent the windscreen of the vehicle,ensuring that the driver's attention is not distracted from the road.There is a risk that sound from another direction will tempt the driverto look towards the source of the sound in a conversation, and thusdivert attention from the road ahead, and the mirror unit according tothe invention avoids this happening.

In FIG. 7, the casing 61 contains a first spring-biased reel 74 whichcarries an electrical lead 75 connected to a microphone 72 provided witha clip 73 to permit it to be clipped on to clothing in a positionwherein it can pick up the user's voice. The reel 74 ensures that, whenthe microphone is released, the lead 75 is wound back into the casing,while permitting it to be pulled out again for use. A secondspring-biased reel 76 carries a lead 77 connected to an earphone 71which the user can pull out and insert into an ear for private use ofthe device. The second reel 76 is arranged to switch off the actuatorconnected to the mirror 60 so that only the user can hear the returningaudio signal. It will be appreciated that a single reel mechanism may beprovided to control release and re-winding of the separate leads or,preferably, a single cable which is split adjacent the free end thereofto carry a microphone and an earpiece respectively.

A push-button 70 is provided in the lower part of the casing 61 toprovide a single operating control for the device operating according tocontext. For example, when the device is in the standby state, pressingthe button will initiate a call, for example, permitting voice-actuateddialling, while when a telephone call is received, pressing the buttonwill answer the call. It may be possible to provide for other functionsto be accessed by pressing the button, perhaps by means of multiplepresses, or even by means of a voice menu system, with a single push ofthe button 70 being used to select an item from the menu. In this way,the driver of a motor vehicle can control the audio device without theneed to take his or her eyes from the road.

FIG. 8 illustrates the interior of a motor car, in which a magnetostrictive actuator 80 is attached to the windscreen 81, for example byadhesive or by temporary attachment means such as a suction cap, andcouples acoustic energy into the windscreen so that it acts as aloudspeaker to radiate sound into the car. The actuator 80 is connectedto an audio signal source such as a cellular telephone or a car radio ormusic player by means of a cord 82, or by a short-range radioconnection. It will be appreciated that the windscreen is only one ofmany panels within a vehicle which could be used to radiate sound. Forexample, an interior door panel or roof panel could be used, or a parcelshelf.

FIG. 9 illustrates a communications system for a building, for examplepermitting speech transmitted from one room to be heard in a selectedone of the other rooms, or in all rooms. The system comprises a basestation 90 located in one room 91, and a receiver 92 mounted on thewindow 93 in each other room 94. The base station 90 comprises amicrophone and means for transmitting a radio signal carrying addressingcoding indicating which of the receivers is to be addressed (for ageneral announcement, this may be all receivers, or a selected group ofreceivers). Each receiver 92 includes a radio receiver and amagnetostrictive actuator, for example in accordance with the invention,arranged within the receiver so as to couple acoustically with thewindow to cause the window to radiate sound into the room. It will beappreciated that each room could also be provided with a base stationpermitting two-way communication.

FIG. 10 illustrates another embodiment of the communications system, inwhich a base station 100 includes an audio signal source, which may be amicrophone or a storage medium (solid-state memory, tape, CD, or other),and a short-range radio transmitter, for example of the type known underthe trade mark Bluetooth. Each remote unit 101 is in the form of aclipboard 102, which can conveniently carry a notepad 103 or printedinformation leaflets, having a radio receiver and transducer device 104mounted on the rear face thereof The device 104 receives the signaltransmitted by the base station 100 and coverts this to an audiofrequency drive signal which is fed to a magnetostrictive transducer, ofthe type disclosed and claimed herein, connected to the board so as tocouple acoustic energy into the board, thereby causing it to act as aloudspeaker, pen-nitting the audio signal to be heard in the vicinity ofthe board, but not at a sound volume that will significantly interferewith the listening of others nearby.

FIG. 11 shows an advertising or like display device comprising anadvertising display panel 110 having detector means 111 mounted thereon,for example a passive infra red detector, to detect the presence of aperson 112 adjacent to the panel. In response to detection of the person112, the detector means signals to an announcement device 113 whichcauses a recorded message to be played via a magnetostrictive actuatorcoupled to the rear face of the panel. The actuator is suitably of thetype described herein, being small and cheap to manufacture, whilegiving acceptable sound quality and volume. The announcement may enhancethe message displayed on the panel, or give location or otherinformation.

The helmet shown schematically in FIG. 12 is, for example, a firefighter's helmet (it will be appreciated that the Figure does not seekto provide an accurate representation of any particular such helmet).The helmet comprises an outer hard shell 120 and inner padding orwebbing 121 to space the wearer's head from the shell and to render thehelmet comfortable to wear. A magnetostrictive actuator 122 is mountedin the space between the wearer's head and in contact with the outershell 120 so as to couple acoustically with the shell. In this way, thewhole of the shell can be made to radiate an audio signal, for example aradio message, in such a way that the wearer can hear the message whileat the same time hearing ambient sound. The actuator may be connected toan external radio device, but in the embodiment illustrated, a smallradio device 123 is mounted within the helmet and is connected via alead 124 to the actuator 122.

FIG. 13 shows a cap 130 having a headband 131 on which is mounted anaudio signal source 132, such as a radio receiver or a solid-state audioplayer, and a magnetostrictive transducer 133, for example of the typehereinbefore described. The actuator 133 is mounted so that the footthereof makes contact with the wearer's skull and couples acousticenergy into the skull. The wearer therefore hears the audio signal bybone conduction, the signal being substantially inaudible to anyoneelse. Batteries to power the device may be mounted on another part ofthe headband.

It will be appreciated that, while the arrangement illustrated in FIG.13 is a simple cap, it is equally applicable to other types of headgear,for example protective helmets, especially for use in noisy environmentswhere ordinary loudspeaking devices or headphones may be ineffective, orwhere it is important that the audio signal should not be overheard byanyone else.

1. A helmet or protective hat having an audio frequency actuatoracoustically coupled therewith, and means for supplying an audiofrequency drive signal to the actuator.
 2. A helmet or protective hataccording to claim 1, wherein the audio frequency actuator is amagnetostrictive actuator.
 3. A hat comprising a headband carrying anaudio frequency actuator connected to means for supplying an audiofrequency drive signal thereto, the actuator being positioned so as toengage the wearer's head, in use, to couple acoustic energy directlyinto the wearer's skull.
 4. A helmet or protective hat according toclaim 2, wherein the means for supplying an audio frequency signalcomprises a radio receiver.
 5. A hat of claim 3, wherein the means forsupplying an audio frequency signal comprises a radio receiver.